A typical expander tool or mandrel has a tension rod and a pusher beam that extend axially into the tube to be straightened. Interengaging wedge jaws on the rod and beam can be shifted axially together to expand the mandrel and thereby radially outwardly deform the tube. The tube is straightened by exerting radial forces on it in regions deviating from an axial centerline.
During the manufacture of tubes made of plate first bent into a round split tube and then welded according to the UOE (U-shaped, O-shaped, Expanded) method, the compression molding method, the three-roller bending method, or the like, the tube material is subjected to very high thermal stresses during the continuous longitudinal seam welding. Such a tube manufactured in this manner normally deviates from a straight shape during the bending process and is subsequently warped during the welding step. Thus a separate straightening step must be carried out to produce the desired straight pipe. Such straightening is particularly required with small-diameter tubing, that is with a diameter less than about 20 inches and a wall thickness greater than about 15 mm.
Thus the tubes are straightened using expander heads or mandrels such as described in GB 1,454,299. The actual expander mandrel consists of a head whose side faces are formed by an array of wedge-shaped jaws spread by a frustoconical outer surface of a core of the head. Relative axial shifting of the jaws, typically braced by the beam, and the head, typically carried by the tension rod, spreads the jaws and expands the tube. In this manner, tubes having a length of up to 18 m are expanded and calibrated gradually.
During expansion of the tube, however, asymmetry (canting) is created at the expander. This has a limited straightening effect, particularly since the material plastically deforms during the expansion. The effect however is minimal and is reproducible in a limited manner only. Hence devices are used together with the expander mandrel that have straightening saddle blocks or rollers axially offset from the expander head or tool and bearing on the tube from the exterior. They apply a radial force to the tube and bend the tube to straighten it by the effect of the expander mandrel. This process, however, applies torque to the pusher beam and tension rod that connect the expander mandrel to an actuator that shifts the mandrel axially along the tube. This has a negative effect on the service life of the components of the expander. Adding to the problem is the fact that the pusher beam, which is particularly thin at smaller diameters, acts as a soft spring, which has an adverse effect on the straightening result.